Turns a unit into its negative by inverting the boundaries. For example,
outsideUnit unitViWord will be the unit of spaces between words. For units
without boundaries (Character, Document, ...), this is the identity
function.

Separator characters (space, tab, unicode separators). Most of the units
above attempt to identify words with various punctuation and symbols included
or excluded. This set of units is a simple inverse: it is true for whitespace
or separators and false for anything that is not (letters, numbers, symbols,
punctuation, whatever).